Two general processes for polymerizing tetrafluoroethylene (TFE) to make non-melt processible polytetrafluoroethylene (PTFE) are known—(1) dispersion polymerization and (2) suspension polymerization. Each process produces polymer with distinctly different properties.
Polymer particles isolated from dispersion polymerization are commonly referred to as fine powder and have the characteristic property of fibrillating under shear stress. The powders can be fabricated into articles by a lubricated extrusion process known as paste extrusion with subsequent removal of the lubricant. The extrudate typically is processed further by heating the extrudate above the melting point of the polytetrafluoroethylene to sinter the extrudate into a tough, coherent article. Fine powder is not fabricable into articles by the common techniques described below used to process suspension polymerized PTFE.
Polymer particles isolated from suspension polymerization are commonly referred to as granular and are non-fibrillating and cannot be paste extruded. The isolated particles are typically irregular, fibrous and coarse and commonly subjected to various finishing operations such as cutting, or cutting and pelletizing, to obtain resin with better processing characteristics. Articles are commonly fabricated from granular powders by a preform/sintering technique adapted from powder metallurgy. The technique involves creating a preform and compacting at room temperature and then sintering the preform, optionally followed by machining. The sintering stage reduces and/or eliminates the void space by coalescence of the molten PTFE particles. These steps are quite time consuming.
Another important granular processing technique is called ram extrusion, which is a continuous powder sintering extrusion process for the production of continuous tubes and rods. In this process, a polytetrafluoroethylene powder is introduced, by means of an automatic metering device, in repetitive cycles into a tube which has been heated to sintering temperature, is compressed by means of a ram and is moved forward a corresponding distance within the sintering tube each time. Under these conditions, the powder sinters together to form a uniformly extruded shaped article.
Fine powder differs distinctly from granular powder in that it is not ram extrudable and is not fabricable into articles of any significant mass or thickness by the preform/sinter technique.
Efforts have been made to modify suspension polymerized polytetrafluoroethylene to make smaller, rounder raw polymer particles suitable for use in ram extrusion without the necessity of further pretreatment, such as cutting and pelletization. Such modification includes the addition of low levels of a comonomer of perfluoro(alkyl vinyl ether) (PAVE), where the perfluoro(alkyl) group is 1 to 3 carbon atoms as provided, for example, in British Patent Specification 1,116,210 issued May 24, 1967. Although levels of up to 1.5 mol % (4.3 wt %) of perfluoro(propyl vinyl ether) (PPVE) are postulated, the highest level of PPVE exemplified is 0.28 weight percent.
Heretofore, it has been believed by those skilled in the art that obtaining high molecular weight PTFE of a sinterable magnitude requires limiting comonomer incorporation. It has been previously demonstrated that copolymerization of TFE with high levels of fluorinated vinyl ether, even in the absence of chain transfer agent, results in a low molecular weight, melt processible polymer. Use of chain transfer agent in melt processible TFE/PAVE copolymerization imparts desirable secondary attributes, namely decreased unstable end groups and narrowed molecular weight distribution. As exemplified in Example 1 of U.S. Pat. No. 3,635,926 to Gresham et al., even when initiator is reduced to the level where chain transfer agent is required to lower molecular weight (thus enabling the acquisition of the desired secondary benefits of chain transfer agent), the polymer produced by copolymerization in dispersion aqueous polymerization is still melt processible and not sufficiently high in molecular weight to be sinterable.
What is desired is an improved granular resin that can be more easily processed and produce sintered articles with equal or improved mechanical properties.